1. Field of the Invention
This invention relates in general to fuel cell powers sources, and more particularly to a method and a system for operating a fuel cell power source.
2. Description of the Related Art
In recent years, as portability has increased in popularity, designers of electronic devices continue to reduce the device's size and weight. These reductions have been made possible, in part, by the development of new battery chemistries such as nickel-metal hydride, lithium ion, zinc-air, and lithium polymer, which enable larger amounts of power to be packaged into a smaller container. Secondary or rechargeable batteries need to be recharged upon depletion of their electrical capacity. Recharging is typically performed by connecting the battery to a battery charger that converts alternating current to a low level direct current of 2 to 12 volts. The charging cycle lasts a minimum of 1 to 2 hours, and more commonly lasts for 4 to 14 hours. One drawback of the current battery technology is the need for sophisticated charging regimens and the slow charging rates.
Fuel cells are expected to be the next major source of energy for portable electronic products. Fuel cells catalytically convert hydrogen molecules into hydrogen ions and electrons, and then extract the electrons through a membrane as electrical power, while oxidizing the hydrogen ions to H2O and extracting the byproduct water. One advantage of fuel cells is the ability to provide significantly larger amounts of power in a small package, as compared to a conventional battery. Their potential ability to provide long talk-times and standby times in portable communication device applications are motivating the continued miniaturization of fuel cell technologies. For example, the Polymer Electrolyte Membrane (PEM) based air-breathing, dead-ended fuel cells are ideally suited for powering portable communication devices and other portable electronic devices.
In the case of a conventional battery powered electronic device, the operational characteristics and usage pattern of the electronic device do not significantly impact the efficiency, the reliability or the lifetime of the battery. On the other hand, when a fuel cell system is used as the power source of an electronic device, many of the fundamental physical, electrochemical and electrical characteristics of the fuel cell system are altered, either permanently or temporarily by the usage pattern of the electronic (load) device. This alteration of the fuel cell system characteristics has a direct impact on the performance and useful life of the fuel cell power source. The average dynamic and peak load patterns of the electronic device also affect the fuel consumption and conversion efficiency of the fuel cell system. The current generation of digital, multi-functional electronic devices has variable duty cycles consisting of sharp short-duration power spikes followed by longer periods of low power needs. Optimizing a fuel cell power source for this class of electronic devices is a complicated process involving keeping track of usage patterns of an individual user, the dynamic power requirements of the electronic device itself, and the operating characteristics of the fuel cell system.
Current technology addresses some aspects of this problem as it relates to automotive vehicles that use a hybrid power source consisting of a battery and a fuel cell system. For example, U.S. Pat. No. 6,321,145 issued Nov. 20, 2001 to Rajashekara, and titled “Method and apparatus for a fuel cell propulsion system” teaches a method for selectively using power either from the battery or from the fuel cell system depending on the current operational context of the vehicle. Similar methods and apparatus have also been described in U.S. Pat. No. 5,808,448 issued Sep. 15, 1998 to Naito, and titled “Method and apparatus for operating an electric vehicle having a hybrid battery”.
Though the current technology methods address the problem of load sharing between a fuel cell and a battery as they relate to hybrid power sources, they do not address the core issue of optimizing the operational performance of a fuel cell power source based on the dynamic power requirements of the electronic device. In addition, these schemes also do not provide for performance effects of the usage profile of the load device on a fuel cell based power source.
Accordingly, what is needed is a method and apparatus that takes into consideration and balances the power characteristics of the fuel cell system, the dynamic load requirements of the electronic device and the usage profile of one or more device user's for use of fuel cell system as a power source for a wide range of load devices.